Chromium stripping bath



Patented June 21. 1932 UNITED STATES PATENT OFFICE 4 JOHN F.MCC'ULLOUGH, OF DETROIT, MICHIGAN, ASSIGNOR 'I'O 'IERN STEDT MANUFAC-TURING COMPANY, OF DETROIT, MICHIGAN, A CORPORATION OF MICHIGAN cnaomuusrairrme BATH- have to be plated over again. It has been customary touse a caustic soda solution bath in doing this stripping, but it hasbeen unlformly found necessary after the stripping to in some way workthe nickel surface underneath, as by bufling, in order to prepare it foragain receiving a deposit of chromium. In short, the nickel-platedsurface is passive, and this I believe is due to the presence on thenickel. surface of an invisible oxide formed by the alkali chromate ion,Cro At any rate, the

surface after such a stripping action is substantially passive, and ifit does receive any deposit of chromium this chromium is quite likely topeel oif.

- It is the object of my discovery to provide a stripping bath whichwill successfully remove the chromium and will not render the undersurface passive to a second deposit of chromium. This I accom'plish byusing a conducting acid aqueous solution which has the characteristic ofreducing the chromic acid formed to a trivalent chromium. This-preventsthe forming of the alkali chromate ions ness of the under. surface;

I find that substantially any of the organic acids which is a goodelectric conductor, and which is formed of hydrogen, carbon, and oxygen,and which is capable of reducing the CrO ,'the-h'exavalent form. ofchromium, to a trivalent form, will answer the purpose. I find thatoxalic acid, tartaric acid, and citric which I believe to be the causeof the passiveacid are most useful in this connection. For

instance, with oxalic acid I preferably use an aqueous solution of -120grams per liter of water. The temperature range should be from, 77 to.degrees Fahrenheit; voltage, 12 volts in full load, using the article tobe stripped as the anode. The chromium then me: serial No. 844,918.

dissolves in its hexavalent'state as chromic acid, and is thenimmediately reduced to the trivalent form by the oxalic acid, and thusthe nickel plate is not rendered passive by the hexavalent chromium ion.The. reaction is as follows: At the anode Cr+3O:=Gr0 chromic acid; usingoxalic acid, H C O we have the reaction:

201'0; (hexavalent)+6H:Cz0|-Cr|(0zO4)3-H3CO;+6H|O.

Using tartaric acid, C H O. the reaction is as follows:

20150:(hflX8V8l0nt)+24Ht0t= 2Cr+++(trivalent)+4HzO0:+400:+2H:0. Usingcitric acid, H C H O the reaction is as follows:

smegma-20:0, (hexava1ent)='= icr+++(trivalent)+ooo,+omo+e(cm)2co 00am.Qxalic acid is cheaper than citric or tartaric acid, and also a betterconductor of electricity.

I might add that'hydrochloric acid has been used in stripping, butthisattacks' the ,base metal, such as die-cast metal, and it is alsomuch slower than oxalic acid. 1 What I claim is: I

1. An electrolytic bath for stripping chromium plated articles byplacing the article at the anode, comprising an aqueous solution of anorganic electrolytic jacidic reducing agent. M

2. An electrolytic bath for stripping chromium plated articles, with thearticle acting as the anode of the bath, which'bath comprises an aqueoussolution of an organic acid capable of reducing chromium from thehexavalent to the trivalent form, and forming a good conductor ofelectricity, said solution being a constituent of a class consisting ofoxalic acid, tartaric acid, and citric acid.

3. An electrolytic bath for stripping chro-' mium plated articles, withthe article as an anode, comprising an aqueous solution of oxalic acid.1.

4. An electrolytic bath for stripping chr6- mium plated articles, withthe article acting as an anode, comprising an aqueous solution of oxalicacid of from 60-120 grams of the acid per'liter of water, and with atempera; ture range of from 77 degrees Fahrenheit of approximately; 12volts.

2 1,aos,soa

to 85 degrees Fahrenheit, and with a voltage 5. An electrolytic birthfor anodically stripping 'a chromium plated article, comprising i I anaqueous solution of a substantial quantity of an organic acidelectrolyte that is substantially passive to the base metal of thesaidchromium plated article.

In testimony whereof I have aflixed my signature.

J OHN F. MGCULLOUGH.

